Drivers for solid state light applications e.g. LED modules or retrofit lamps have to comply with mains harmonics regulations which may be defined by national or international standards, energy labels, the mains power provider, etc. Some of the standards are optional, but others are mandatory, because otherwise the product (light source including the driver) may not be released to the market in a certain region. Different kinds of power factor correction techniques are used to limit harmonics of the input current and to reduce the idle current drawn from the mains. In the optimal case, a sinusoidal input current is drawn from the mains, providing a sinusoidal supply voltage. This results in a high power factor and no harmonics, whereby an ohmic load is emulated which does not draw reactive power from the mains.
To control the input current, power factor control techniques are known wherein the power stage is a PFC stage controlled by a PFC controller. The PFC controller multiplies the rectified input voltage by a low-pass filtered command, representing an average power demand for driving the PFC stage and for controlling the instantaneous input current.
Alternatively, the driver devices can also be designed to generate a low total harmonic distortion level such that the mains harmonics regulations are fulfilled without requiring a dedicated PFC stage. Such a driver device is known e.g. from US 2010/0060182 A1.
As a further possibility to reduce the harmonic distortion, a passive input current shaper such as an RC or LC circuit may be used on the primary side in combination with a single-stage switch-mode power supply. In this case, the control loop of the switch-mode power supply results in a regulation between the bus voltage and the transferred power and is used to control the discharge of the electrical energy on the primary side which influences the harmonics during recharging of the primary capacitor.
As a further alternative solution, a linear driver such as a tapped linear driver (TLD) or switch matrix driver (SMD) may be used, wherein the current provided to the total LED load is controlled and power is guided in a certain manner into the segments, forming the total LED load, and wherein the shape of the input current is determined by the current into and the arrangement of the total LED load.
The limitation of the drivers known from the prior art is that the power delivered to the LED load matches the requirement as long as the mains voltage corresponds to the expected parameters, such as wave shape and amplitude. Control loop and parameters are designed to provide sufficient power to the load while maintaining the allowed input current harmonics. However, even if the mains signal is distorted, this control loop and these parameters are still applied. As a consequence, e.g. during a partial mains dropout or flat topped mains, the power delivery will fall below the average required power, because the control loop and parameters are designed for a nominal input signal. As a result, the driver devices known from the prior art cannot provide continuous stable power to the load if the mains voltage is distorted, even if the power train would allow for delivering sufficient output power.